Answer
$37a^2-23ab+35b^2$
Work Step by Step
Using $a(b+c)=ab+ac$ or the Distributive Property, the given expression, $
12a^2-3ab+5b^2-5(-5a^2+4ab-6b^2)
$, is equivalent to
\begin{array}{l}\require{cancel}
12a^2-3ab+5b^2-5(-5a^2)-5(4ab)-5(-6b^2)
\\\\=
12a^2-3ab+5b^2+25a^2-20ab+30b^2
.\end{array}
By combining like terms, then this simplifies to
\begin{array}{l}\require{cancel}
(12a^2+25a^2)+(-3ab-20ab)+(5b^2+30b^2)
\\\\=
37a^2-23ab+35b^2
.\end{array}
